1. Field of the Invention
This invention relates to a method to reduce thermal conductivity of nitrides, while at the same time keeping electrical conductivity high.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
III-V nitride is a wide band gap semiconductor, and therefore remains a good unipolar semiconductor even at high temperatures above, e.g., around 1000 degrees Kelvin (K). Furthermore, even at higher temperatures above 1000 K, the nitride semiconductor remains unipolar. This property makes the III-V semiconductor a promising material for use in thermoelectric devices. Generally, electrodes based on metal materials degrade at temperatures above about 1000 K, but some electrodes for nitrides still operate adequately at the high temperatures.
However, the nitride semiconductor's thermal conductivity is too large to use in a thermoelectric device (220 Watts per millikelvin (W/mK) measured by [1]). The application of III-V nitride films to thermoelectricity has been quite limited so far, and there is little documented research available in this area [2-4]. Because of the high thermal conductivity of nitride, nitride has not attracted attention. The thermoelectric performance can be described by the figure of merit, ZT=α2σT/κ, where α, σ, κ, and T, are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively.
For example, the long arrow 100 in FIG. 1 represents that AlInGaN 102 has good thermal conductivity, 220 W/mK at 300 K, along the direction of the long arrow 100, when the ends 104, 106 of the AlInGaN 102 are at temperatures T1 and T2, respectively, and the long arrow 108 represents the AlInGaN has good electrical conductivity along the direction of the long arrow 108. For this case, ZT is small.
This contrasts with FIG. 2, wherein the arrow 200 represents that a good thermoelectric material 202 has bad thermal conductivity (comparatively short arrow 200 as compared to arrow 100) along the direction of the short arrow 200 when the ends 204, 206 of the material 202 are at temperatures T1 and T2 respectively, yet maintains good electrical conductivity (as represented by the longer arrow 208) along the direction of the longer arrow 208. For this case, ZT is higher compared to the case shown in FIG. 1.
FIG. 3(a) and FIG. 3(b) illustrate basic thermoelectrics based on the Seebeck effect, wherein a temperature gradient ΔT=T1−T2 (T1>T2) across the ends 300, 302 of a semiconductor 304 generates electromotive force, leading to thermopower (an induced thermoelectric voltage ΔV) across the ends 300, 302 of the semiconductor 304, which can be used to drive (supply power to) a load R. As a result of the temperature gradient ΔT, end 300 is at temperature T1 and voltage V1, and end 302 is at temperature T2 and voltage V2.